In communication systems, methods for detecting and correcting errors that occur during transmission are used to increase reliability. One possibility is to use Low-density parity-check (LDPC) code which is described in [1] and is a capacity-approaching coding scheme which has attracted a lot of attention nowadays. Compared with turbo code, which is also a capacity-approaching code, LDPC code has better performance in case of a large block size even though the decoding complexity is less that that of turbo code.
Another commercial advantage of LDPC is that the LDPC patents have expired, so companies can use LDPC without having to pay for intellectual property rights.
The usage of an LDPC encoder together with an M-ary modulator, termed as LDPC coded modulation, is described in [2] and provides for high power and bandwidth efficiency. The M-ary modulator maps every log2 M LDPC encoded bits to a constellation symbol, for example according to Gray mapping. This mapping does not introduce any latency in transmission, which is different from bit interleaving in bit-interleaved coded modulation (BICM). By optimal design of LDPC code, LDPC coded modulation can get close to the capacity limit of coded modulation.
The LDPC code and LDPC coded modulation have been widely introduced into next-generation wireless communication systems for reliable and efficient transmission at high data rates. LDPC in combination with quadrature amplitude modulation (QAM) and MIMO-OFDM (multiple input multiple output-orthogonal frequency division multiplexing) has been shown to be superior over turbo code in high-throughput (HT) wireless local access networks (WLANs). It is also a competitive candidate in the optional HT-mode in proposals for the IEEE 802.11n standard. LDPC code has also been proposed for IEEE 802.16, standard for Broadband Wireless Access (BWA), which also requires higher-order modulation. There are also a lot of proposals on the design of LDPC into the ultra-wideband (UWB) wireless communications, where there is possibility that LDPC is used in a QAM UWB system.
In the conventional bit-interleaved coded modulation (BICM) schemes applying convolutional code or turbo code, a bit interleaver between the encoder and modulator is necessary in order to permute coded bits to different positions in different symbols.
In IEEE 802.11a standard, the bit interleaver processes two steps of permutation by
Step 1:
Mapping adjacent coded bits onto nonadjacent modulated symbols to avoid burst errors introduced by single symbol and channel correlation.
When N denotes the codeword size, the first permutation is defined asi=(N/d)(k mod d)+floor(k/d), k=0,1,K,N−1,  (1)where d is the interleaving depth, and floor(•) denotes the largest integer not exceeding the parameter.Step 2:
Mapping adjacent coded bits alternately onto more significant bits (MSBs) and less significant bits (LSBs) of the constellation to avoid long runs of LSBs with low reliability.
The second permutation is defined asj=s×floor(i/s)+(i+N−floor(d×i/N))mod s, i=0,1,K,N−1  (2)where
  s  =      max    ⁡          (                                                  log              2                        ⁢            M                    2                ,        1            )      is determined by the modulation order M.
Two methods for use with LDPC coded modulation are described in the following.
The first method is to use the bit interleaver according to IEEE 802.11a as described above to permute the LDPC encoded bits, cf. [2].
The second method is the direct mapping of a LDPC codeword into an M-ary symbol. This is explained in the following.
An (N, K) LDPC code is given by its parity-check matrix, H=[H1 H2], where K and N are the numbers of information bits (to be coded) and the number of LDPC coded bits, respectively. The generator matrix is given byG[IH1TH2−T]  (3)where Ik×k is the identity matrix generating systematic bits of the codeword. The codeword based on G isc=uG=[uuH1TH2−T]=[up],  (4)where u is the information sequence (vector of user data bits). The codeword c consists of the vector of systematic bits u and the vector of parity bits p.
According to the second method, consecutive log2 M bits from codeword c are directly mapped into an M-ary constellation symbol.
The usage of the first method, i.e. using a bit interleaver, is not optimal since in contrast to conventional BICM, a bit interleaver is not required in LDPC coded modulation. This is the advantage of LDPC coded modulation over conventional coded modulation.
Further, the first method has the disadvantage that it is not optimal to apply the permutation rule used in IEEE 802.11a for LDPC coded modulation. This is because the second permutation according to equation (2) maps adjacent coded bits to more significant bits (MSBs) and less significant bits (LSBS) alternately in phase shift keying (PSK) and QAM, which is only suitable for non-systematic codes such as convolutional code that is used in according to IEEE 802.11a (see [3]).
The second method has a similar drawback. While consecutive coded bits from codeword c are mapped directly into PSK or QAM symbols, the systematic bits and parity bits are placed into MSBs and LSBs evenly.
In [4] the result of a study is given that shows that turbo code performs better when systematic and parity-check bits are assigned different power levels than when equal power is assigned to the whole codeword. For large block size, better performance is achieved when the systematic bits are protected better than the parity check bits and this effect becomes stronger as the block size increases. Research on unequal error protection in LDPC code mainly focuses on the construction of the LDPC codeword (cf. [5], [6]). It has been found out that in general the larger the variable node degrees, the higher the average radiabilities after decoding. This conclusion is only applicable to irregular LDPC code, however.
An object of the invention is to increase performance of LDPC coded modulation methods.
The object is achieved by the method of transmitting data, the method for receiving data, the transmitter, the receiver, and the computer program products with the features according to the independent claims.